1. Field of the Invention
The present invention relates to a lens assembly used for an optical recording apparatus. The present invention also relates to an optical head and an optical disk apparatus which incorporate such a lens assembly. In this specification, the "optical disk apparatus" refers not only to an apparatus used strictly for reading data from an optical disk, but also to a magneto-optical disk apparatus capable of both reading and writing data with respect to a magneto-optical disk by magnetic field modulation or optical pulse modulation. The phrase "optical disk apparatus" further refers to other kinds of optical disk apparatus.
2. Description of the Related Art
Generally, in an optical disk apparatus of the above type, the data storage capacity of an optical disk is in inverse proportion to the square of the diameter of a focal spot to be formed on the recording region of the disk. Thus, to increase the storage capacity of the disk, it is desirable to reduce the diameter of the focal spot as much as possible. As is known, the diameter of the focal spot is proportional to ".lambda. (wavelength of laser beam) divided by NA (numerical aperture)." Thus, theoretically, there are two possible ways to reduce the diameter of the focal spot, that is, to reduce .lambda. or to increase NA. However, due to certain limitations concerning the properties of materials, it is rather difficult to obtain a semiconductor laser capable of emitting light whose wavelength is sufficiently small. Thus, for the present, it is impossible to obtain an inexpensive, short-wavelength laser (blue laser) which is satisfactory in terms of output and lifetime. Under these circumstances, the second option (i.e., to increase NA) may inevitably be employed to reduce the diameter of the focal spot, and new techniques to achieve a smaller diameter have been eagerly sought after.
In this connection, reference is now made to FIG. 17 showing a conventional arrangement adopted for obtaining an increased NA (see JP-A-8-221790). In the illustrated conventional device, use is made of a lens assembly including two objective lenses, that is, a first objective lens L1mounted on a lens actuator A and a second objective lens L2mounted on a slider S which in turn is supported by a carriage. The second objective lens L2is arranged adjacent to the surface of a transparent disk substrate Dl of an optical disk D. With such an arrangement, the overall NA of the lens assembly can be made greater than when only a single objective lens is used.
Though the overall NA is increased to a certain extent, the above conventional arrangement is disadvantageous in the following point. As stated above, the second objective lens L2is arranged adjacent to (but, outside of) the transparent substrate D1of the disk D, and this lens (together with the first objective lens L1) causes the laser beam to converge at a point in the recording region D2of the disk D. With such an arrangement, the NA, which can be expressed by "n (refractive index of the transparent substrate D1).times.sin .theta." as shown in FIG. 17, cannot be made greater than 1. Practically, the NA is about 0.8 at most.
The improvement of the data storage capacity of a magneto-optical disk may be achieved by another method disclosed in JP-A-10-124943 for example. According to this conventional method, part of the focal spot of the laser beam is magnetically masked by utilizing the fact that magnetic layers laminated on a recording layer of the disk have different Curie temperatures.
In the above method, the diameter of the focal spot itself is not reduced. Thus, the problem of crosstalk between adjacent tracks may still be incurred, which is a hindrance to the improvement of the data storage capacity of the disk.